The present invention relates to a composition capable of hydrogen sorption in a closed container at low pressure, and particularly it relates to a composition formed of an unsaturated organic substance and a hydrogenation catalyst.
Getter materials have been in use for a long time in all the industrial applications which require the vacuum maintenance in a closed system. A particularly important application uses the property of low thermal conductivity of the vacuum for realizing thermal insulation systems for any material or device. Said insulation is generally obtained by creating, outside the material or device to be insulated, a double wall with evacuated interspace.
Since hydrogen has, among gases, the largest thermal conductivity, it is particularly important to provide means for sorbing the traces of hydrogen which are still present in the evacuated interspaces so as to complete the achievement of vacuum. Furthermore, due to the small size of the hydrogen molecule, this gas outgases very easily from the walls of the evacuated containers and has to be continuously sorbed in order to maintain the thermal insulation property.
It is known that the organic compounds comprising unsaturated bonds among carbon atoms react with hydrogen in the presence of a suitable catalyst being converted into the corresponding saturated compounds. By virtue of this reactivity, said compounds, combined with a suitable catalyst, can be advantageously used as hydrogen getters.
Although in principle all compounds comprising a double or triple bond between two carbon atoms can sorb hydrogen, some fundamental requirements have to be satisfied for a compound to be industrially used. A first requirement relates to the specific rate of the reaction with hydrogen, which has to be high in order to avoid an accumulation of hydrogen in closed systems. Furthermore, it is necessary that said hydrogenation reaction be capable of occurring also at very low partial hydrogen pressures, in other words that the equilibrium of the reaction be shifted towards the products. Another requirement, important for ensuring that the unsaturated compound remains on the catalyst, is that said unsaturated compound have a low vapor pressure within the whole range of working pressure and temperature.
U.S. Pat. No. 3,896,042 discloses a method for sorbing hydrogen from a closed system at low pressure and low temperature, which consists in placing inside said container a hydrogenation catalyst suitably supported on an inert substrate and coated with an unsaturated organic compound. The unsaturated organic compounds described in said patent are some arylacetylenes and particularly dimerized propargyl phenyl ether, dimerized benzylacetylene, dimerized phenylpropiolate, dimerized diphenyl propargyl ether and polydipropargyl ether of bisphenol-A.
U.S. Pat. No. 4,405,487 describes a combination of getter materials, which can be used for instance inside sealed containers for electronic and mechanical components, comprising a moisture getter and a hydrogen getter. The latter is formed of a hydrogenation catalyst and of a solid acetylenic hydrocarbon, comprising no nitrogen and sulphur heteroatoms. In fact, according to the patent teaching, these elements can bring about the generation of undesired by-products by hydrolysis. The acetylenic hydrocarbon which is indicated as particularly advantageous also from the point of view of the hydrogenation rate and of the hydrogen gettering capacity per gram of compound is 1,4-diphenylbutadiyne.
U.S. Pat. No. 5,624,598 and U.S. Pat. No. 5,703,378 describe a composition for hydrogen sorption at low pressures and high temperatures, which can be used for instance for thermal insulation of the pipes for transportation of high temperature fluids. Said composition is formed of a suitable catalyst and of a hydrocarbon compound, or polymer, comprising triple bonds between carbon atoms and aromatic moieties selected among benzene, styrene, naphthalene, anthracene, diphenyl, fluorene, phenanthrene and pyrene. The presence of aromatic moieties has the purpose of raising the melting temperature of the unsaturated compounds and of their hydrogenated derivatives, so that they are solid at the working temperatures and pressures.
However, a first drawback of the compositions indicated in the last mentioned patents consists in that they are obtained as mixtures of many compounds having different molecular weight. This involves problems in the control and reproducibility of the physical and chemical characteristics of the product. In particular, as it is known, it is difficult to obtain a solution of organic compounds having a very high molecular weight; consequently, the steps for the production of the final getter which require passing through a solution, such as the mixing with the hydrogenation catalyst and the deposition on a porous substrate, are difficult.
A second drawback of the above described composition for hydrogen sorption consists in the high production cost thereof. In facts, the synthesis of the unsaturated compounds or polymers is carried out by a condensation reaction starting from acetylenes and aromatic halides which requires the use of triphenylphosphine and palladium complexes as catalysts. At the end of the reaction, for economical reasons it is necessary to isolate the palladium complex, separating it from the reaction products so that it can be used again. Further, the other catalyst, triphenylphosphine, is a toxic product which should not be used in industrial processes in order to avoid safety and ecological problems.
Therefore, object of the present invention is providing a hydrogen getter composition free from said drawbacks. Said object is achieved by means of a hydrogen getter composition whose main features are specified in the first claim and other features are specified in the subsequent claims.
A first advantage of the hydrogen getter composition according to the present invention consists in that it allows final hydrogen pressures lower than those typical for getters according to the state of the art to be reached with particularly high sorption rates.
A second advantage of the hydrogen getter composition according to the present invention is that its production cost is very low. In fact, the synthesis of the unsaturated organic substances which are the components thereof is carried out from starting materials already available on the market and by means of processes which provide for high yields without using expensive catalysts and do not require subsequent separation steps.